environment by decomposing organic matter. Some of these are toxic to humans but are responsible for making many kinds of antibiotics. These are eukaryotic cells and include yeasts, mushrooms, and molds. Oxygen is necessary for metabolism. They get nutrients via absorption and will undergo sexual or asexual reproduction through spore formation. •
Plantae—these are the part of the life cycle that undergoes photosynthesis and include many different species, including seed-bearing plants and non-seedbearing plants, flowering and nonflowering plants, and vascular and nonvascular plants. We will talk more about plants in a subsequent chapter.
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Animalia—this is the animal kingdom. They are eukaryotic and, while most are aquatic, there are many land animals. They depend on plants and other animals for nutrition. Most will reproduce via sexual reproduction, which involves male and female gametes that are fertilized. Fish, insects, mammals, amphibians, sponges, and worms belong to this category. All require oxygen for metabolism. They acquire nutrients via ingestion. A few will undergo asexual reproduction as part of their life cycle.
ARCHAEA In this section, we’ll talk about archaea as a domain because it hasn’t been discussed before. These are prokaryotic microbes that have no cell nucleus. Previously considered bacteria, they will have different ribosomal RNA and other different features that make them unique among species. There is just one kingdom, called the Archaebacteria and multiple phyla. As you will see, they share genes similar to eukaryotes as well, particularly those involved in the enzymatic control of transcription and translation. While considered extreme organisms, they are also found in more traditional environments, such as marshlands, soil, oceans, and in the human mouth, on the human skin, and in the gut. There are many archaea species in the oceans, where they play a role in the nitrogen cycle and the carbon cycle. They can be commensals or mutualists but tend not to be pathogenic. There are several phyla that have been proposed as already mentioned. The Euryarchaeota and Crenarchaeota are well-known
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